1. Field of the Invention
The present invention relates to a method for printing an object using a digital color image printer. The invention further relates to a print system configured to apply a method for printing an object.
2. Description of the Related Art
Various forms of printing objects are known. Objects relate to three-dimensional bodies in a physical world. Printing objects relates to a layerwise application of material in a similar way as images are printed, thereby composing an object. Most often, printing is in the form of jetting material from a nozzle or from an array of nozzles, but other forms, such as the application of two-dimensional toner layers or color printed paper layers have also been described. Printing may be performed on a substantially flat surface or on a nucleus, or base object, that is manipulated to have material applied from different sides. All these forms of printing and more are comprised in what is known as 3D-printing, or additive manufacturing.
Digital color image printing comprises various processes that convert a digital, electronic image into a reproduction of the image on a receiving material, an image being two-dimensional by definition. In the reproduction, colorants are used to locally generate an optical density in accordance with the electronic image. In some processes, these colorants are applied on top of the receiving material, thereby locally thickening the reproduction. Such processes comprise the application of toner material, UV-curable ink and phase change materials. In other processes, the colorants are absorbed into the receiving material, such as by application of solvent and water based colorants. The processes that locally thicken the material may be used to create stacks of colorant material by applying the colorants in slices on top of each other. Applied in this way, these processes look like three-dimensional processes and may be deployed for printing objects. Nevertheless, the colorants are commonly applied as dots, having a flat, mostly circular shape in order to have a large surface. In some processes, ink drops of 10 pl are applied, resulting in 60 μm dots, having a height of about 3.5 μm. The specific dot shape depends on the ink properties, such as its viscosity, the application temperature and the receiving material, but generally, the dots have a flat shape and quite a number of slices have to be applied to obtain a predetermined height. It is also noted, that in some processes, the colorant material is limited to a predetermined thickness, since a further processing step is needed to obtain a stable state of the applied material. For example, in the process of applying UV-curable ink, a further processing step of curing an applied ink dot by UV-radiation (ultraviolet light) is needed to solidify the liquid material. Thus, a limit exists for the amount of material in the applied dots.
A special kind of object that has been printed with a two-dimensional color image printer, is a relief image. For this object, a height channel is added to the customary color image channels. This enables an image printer to derive a number of slices of colorants to be put on top of each other for each print position, thereby creating an image having palpable height differences. In these prints, a reproduced color of a pixel is made independent of the height of the pixel by adding a uniformly colored, preferably white, ink layer on top of a relief structure as defined by the height information. The colorants for generating a color of a pixel are printed as a skin layer on top of the uniformly colored ink layer in accordance with the color channels of the image. Thus, the colorants are employed in a way they are composed for, since their color properties are optimal when applied on a white background. Another process for making a relief image comprises the use of transparent ink that is printed in accordance with the height information on top of a reproduction of the image on the receiving material, that is usually white. These relief images are objects for viewing from a main direction, similarly to viewing usual images on (locally) flat material.
In contrast, an arbitrary object may be viewed from multiple sides. Therefore, the information for printing an object comprises a definition of the shape of the object and its appearance from several sides, including images on apparent sides. The slicewise material application is derived from this information in the form of image slices that are applied in sequence. Following a similar approach as indicated above, the colorants that determine the appearance of the object are applied as a skin layer onto a white ink buffer layer in order to optimally use the color properties of the colorants. The underlying shape of the object is composed of an arbitrary composition of colorants, since their color is not visible and their application is aimed at begetting a material volume.
Still, it is one thing to derive image slices from an object definition, comprising an image on multiple sides of the object. It is another to obtain a printed object as defined. Without major modification of the colorant materials as applied in digital color printing, it has been found that a problem occurs in obtaining a comparable gamut for colors on all sides of the object. It is an object of the present invention to obtain a similar range of colors for all sides of a layerwise printed object, independent of the direction wherein the image slices are applied.